Kull of Atlantis, aka Kull the Conqueror, is the very first Sword & Sorcery hero invented by Robert E. Howard, although poor reception to Kull's debut story "By This Axe, I Rule" caused Howard to retool the story and the character to create Conan the Barbarian and his debut novel, "The Phoenix on the Sword". Ultimately, Howard did manage to get Kull sold after building up his fame with Conan, but he only published three of Kull's stories before his unfortunate suicide, with the remaining 9 stories and a poem being published posthumously.
Kull is a barbarian tribal from the ancient realm of Atlantis, hailing from around the year 100,000 BC; in the time period he comes from, Atlanteans are little more than semi-Stone Age hunter-gatherers, whilst a continent to the east, named Thuria, houses several Iron Age nations, in the form of Valusia (the most powerful), Commoria, Grondar, Kamelia, Thule, and Verulia. Kull was born into the ranks of a tribe settled in the Tiger Valley, but his tribe was wiped out by a catastrophic flood that swept through the valley when he was a toddler, leaving him as a feral child that spent several years being raised by a tiger. When his tiger-mother was killed by hunters from the Sea-Mountain tribe, Kull was captured and taken in, learning human language and thought. He is ultimately exiled when, during his adolescence, he offers a mercifully quick death to a woman who has dishonored herself and who is about to be burned to death by a mob.
Setting out on his own, Kull attempts to reach Thuria, but is captured by Lemurian Pirates on his way, spending several years as a gallery slave until he is freed by a mutiny. Now somewhere between late adolescence and early twenties, he tries the life of piracy for himself, with his fighting skills and courage seeing him rise to become a pirate captain with his own ship, which gains a fearsome reputation in the seas surrounding Atlantis and Thuria. That comes to an end in a naval battle off the coast of Valusia, when his ship is sunk and Kull is the sole survivor. Cast up on the shores of Valusia, he turns to the outlaw's life, but is soon captured by Valusian guards and imprisoned, where he chooses to become a gladiator rather than be executed.
Being a badass proto-Conan, Kull thrives in the arena, earning great fame, to the point his fans ultimately petition the king and get him successfully released. Deciding that fending for himself is a mug's game, he joins the Valusian Royal Army as a mercenary, hoping to work his way up the ranks. When he is approaching is thirties, he is recruited by King Borna of Valusia to assassinate the ambitious Lemurian sorcerer, Rotath; his successful completion of this mission sees him promoted to the general command of King Borna's mercenary forces. In this place, Kull earns the respect and loyalty of his forces, which serves him well when King Borna's cruelty and despotism sparks off a civil war. Kull decides to seize advantage of the chaos, becoming first leader of the rebels, and then killing Borna to take his throne.
Unfortunately, once he has it, Kull finds that the life of a king is far less grand than he had believed. Internal conspiracies and external threats see him fighting harder than ever for a throne he grows less and less sure he wants, ultimately sinking deep into philosophical musings.
Kull vs. Conan
Although the resemblances between the two is obvious - Conan even traces his ancestry to the Altaneans, and some sources say to Kull himself - Kull and Conan couldn't be more different in some very key ways. Beyond the obvious fact that Kull is... well, a lot more of a loser than Conan, the major differences are introspection and libido. Kull is a broody, melancholic bastard, who spends a lot of time lost in deep introspection, to the point his canonically final story, "The Mirrors of Tuzun Thune", consist entirely of him sitting around and thinking about philosophy. Conan is a man of purpose and action. And whereas Conan is famous for picking up at least one girl in every story, Kull seems all but indifferent to women, and never has a single love interest.